First-principles calculations of phase transition, elasticity, and thermodynamic properties for TiZr alloy

Structural transformation, pressure dependent elasticity behaviors, phonon, and thermodynamic properties of the equiatomic TiZr alloy are investigated by using first-principles density-functional theory. Our calculated lattice parameters and equation of state for α and ω phases as well as the phase transition sequence of α → ω → β are consistent well with experiments. Elastic constants of α and ω phases indicate that they are mechanically stable. For cubic β phase, however, it is mechanically unstable at zero pressure and the critical pressure for its mechanical stability is predicted to equal to 2.19 GPa. We find that the moduli, elastic sound velocities, and Debye temperature all increase with pressure for three phases of TiZr alloy. The relatively large B/G values illustrate that the TiZr alloy is rather ductile and its ductility is more predominant than that of element Zr, especially in β phase. Elastic wave velocities and Debye temperature have abrupt increase behaviors upon the α → ω transition at around 10 GPa and exhibit abrupt decrease feature upon the ω → β transition at higher pressure. Through Mulliken population analysis, we illustrate that the increase of the d-band occupancy will stabilize the cubic β phase. Phonon dispersions for three phases of TiZr alloy are firstly presented and the β phase phonons clearly indicate its dynamically unstable nature under ambient condition. Thermodynamics of Gibbs free energy, entropy, and heat capacity are obtained by quasiharmonic approximation and Debye model.